Thursday, December 01, 2011

New Recipe for Rare Earth Magnets

Researchers from the Idaho National Engineering and Enironmental Laboratory (INEEL), Ames Laboratory and Brookhaven National Laboratory have developed a new recipe for rare earth magnets, the kind found in computer hard drives, cordless power tools and miniature speakers. The secret ingredient, they say, is nonmetallic crystals of titanium carbide, which makes for stronger, more durable magnets that are less sensitive to heat and so less expensive to manufacture.

INEEL materials scientist Dan Branagan and his collaborators started with the standard mix of rare earth metals--neodymium, iron and boron--and added titanium and carbon. They combined the elements into a melt, cooled it into an ingot and then remelted the ingot, feeding the liquid into a pressurized stream. The stream shot the metal onto a copper wheel spinning 100 miles per hour, thereby creating a metallic glass ribbon. (Another option for this step is forming an ash from the glass.) When they reheated the glass to about 600 degrees Celsius, the glass blobs--instead of grouping together into larger crystals--separated into nanoscopic metallic and nonmetallic ones.

The trick here is that the titanium carbide crystals get in between the metal crystals and prevent them from growing bigger. Smaller crystal grains means each one can contain only one magnetic field, or domain--and fewer domains per grain minimizes defects. When more domains are aligned, it raises the strength of the composite material's magnetic field. As an added bonus, because the nonmetal crystals effectively limit the size of the metallic ones, the process is far less sensitive to heat. "That means lower costs and less stringent controls," Branagan says. An error of 10 degrees in current manufacturing methods can lead to grains that are too large.